Flexography utilizes resilient relief members to transfer an image from a printing member to a recording medium. As in letterpress printing, a flexographic member or plate has a surface comprising an “imagewise” pattern of raised features. Ink is applied to and carried by these raised features and transferred therefrom to the image receiver. Although developed primarily for printing packaging materials, flexography is today used in a wide variety of applications and on recording media such as paper, corrugated board, films, foils, and laminates.
Flexographic printing plates can be prepared from photosensitive elements comprising a photopolymerizable layer containing an elastomeric binder, a monomer, and a photoinitiator, interposed between a support and a cover sheet or multilayer cover element. A standard process of making such photosensitive elements is described in U.S. Pat. No. 4,460,675; as set forth therein, a previously extruded photopolymerizable composition is fed into the nip of a calender (i.e., a series of hard, high-pressure rollers in rolling contact) and is pressed between a support and a multilayer cover element to form a photopolymerizable layer. Upon imagewise exposure of the photosensitive element to actinic radiation through a photomask, the exposed areas of the photopolymerizable layer become insolubilized. Treatment with a suitable solvent or solvent mixture removes the unexposed areas of the photopolymerizable layer, leaving a printing relief which can be used for flexographic printing. See also U.S. Pat. Nos. 4,323,637, 4,427,759, and 4,894,315.
A common technique for bringing a photosensitive element and a photomask into close contact with one another is to draw a vacuum between them, usually by means of a vacuum frame. Digital methods and associated recording materials that do not require a separate photomask have also been developed; see, e.g., PCT Appl. Nos. WO 94/03838, WO 94/03839, and WO 96/16356. Such recording materials comprise a conventional photopolymerizable layer, as described above, and additionally a layer capable of forming an integrated photomask. The additional layer is sensitive to infrared (IR) radiation but opaque to actinic (e.g., ultraviolet (UV)) radiation. This IR-sensitive layer may be imaged digitally, whereby the IR-sensitive material is imagewise vaporized or transferred to a superposed film. Subsequent overall exposure of the photopolymerizable element to actinic radiation through the integrated photomask produces an imagewise pattern of hardened regions; unpolymerized areas that did not receive exposure, and remaining areas of the IR-sensitive layer, are washed away. Following drying, the flexographic printing plate is ready for use.
Developing the exposed photosensitive element with a solvent or solvent mixture is time-consuming, since drying for extended period (0.5 to 24 hours) is typically necessary to remove entrained developer solution. In addition, these developing systems produce potentially toxic by-product wastes (both the solvent and any material carried off by the solvent) during the development process. For printing of food packaging, elimination of solvent residue on the finished plate is also crucial.
To avoid these problems, a “dry” thermal development process may be used. In this approach the photosensitive layer, which has been imagewise exposed to actinic radiation, is brought into contact with an absorbent material at a temperature sufficient to cause the unexposed portions of the photosensitive layer to soften or melt and flow into the absorbent material. See, e.g., U.S. Pat. Nos. 3,264,103, 5,015,556, 5,175,072, 5,215,859 and 5,279,697. A photosensitive silver-halide film target in a vacuum frame is imagewise exposed, and the exposed portions thereof layer remain hard at the softening temperature for the unexposed portions. The absorbent material collects the unexposed and softened material, and is then separated and/or removed from the photosensitive layer. The cycle of heating and contacting the photosensitive layer may need to be repeated several times in order to sufficiently remove the flowable composition from the unirradiated areas and form a relief structure suitable for printing. The resulting raised relief structure of irradiated, hardened photopolymer represents the desired printing image.
Finally, the flexographic printing plate prepared by any of the above-described processes (that is, having the imagewise relief pattern) may be post-exposed and/or chemically or physically after-treated in any sequence to detackify the surface. For example, UV radiation with a wavelength not longer than 300 nm may be used for post-exposure for detackification.
Another type of flexographic plate is produced from a liquid photopolymer. In a typical production process, a clear plastic protective cover film is mounted over a transparency having a negative version of the image to be printed. The transparency is placed emulsion-side up on an exposure unit that emits actinic radiation. A motorized carriage then deposits a layer of liquid photopolymer over the transparency and cover film. The carriage ensures that the liquid is deposited evenly over the cover film and at a controlled thickness. As the liquid is deposited, the carriage also places a substrate sheet over the liquid. The substrate sheet is specially coated on one side to bond with the liquid photopolymer and to serve as the back of the plate after exposure. Initially, the entire substrate side of plate is exposed to actinic radiation. This exposure hardens a thin base layer of the liquid photopolymer and causes it to adhere to the plate substrate. A second exposure through the negative forms the image on the plate. As with sheet materials, the image areas are hardened by this exposure while the non-image areas remain liquid. The hardened photopolymer, adhered to the substrate, is removed and subjected to chemical processing (i.e., washing with solvent or water-based fluids) to remove unwanted liquid in the non-image areas, leaving raised image areas. A post-exposure cure hardens the whole plate. In some systems, uncured photopolymer is reclaimed for reuse.
While this type of flexographic plate is conveniently prepared, ecologically deleterious waste is ordinarily produced during processing. In addition, the image quality may be inferior to that of digitally produced flexographic plates due to the cover film between the image and the photopolymer; the thickness of the film produces higher dot gain and lower tonal resolution. Accordingly, there is a need for techniques for producing liquid-photopolymer flexographic plates with reduced production of waste and without an intermediate cover film.